In a linear motor used for table feeding in an actuator of a machine tool or transporting equipment, if an armature coil of a rotor is shorter in length than an array of permanent magnets (an array of field magnets), because a magnetic circuit of the armature core is not endless, an end effect occurs. Thus, a one-cycle cogging thrust, that is a second-order cogging thrust, is generated in a pitch between the poles of the permanent magnet array. The second-order cogging thrust further generates a fourth- and a sixth-order cogging thrusts that are doubles of a harmonic wave thereof, resulting in an uneven thrust due to the rotor. Furthermore, because a first iron core has a slot in the same manner as in a conventional rotating motor, a slot cogging thrust is also produced. This also results in an uneven thrust.
A so-called magnetic attraction offsetting type linear motor that is a conventional linear motor includes: stators that are arranged side by side in parallel and onto which a plurality of equally spaced permanent magnets, having opposing polarities thereof arranged in an alternating manner along a direction of movement, are fastened on an inner surface thereof; and a rotor having divided cores each of which is wound with an armature coil, arranged in the direction of movement, and mechanically coupled to each other, and is divided and supported along the center of the stators in a movable manner along the direction of the movement, with the divided cores arranged so that the lateral sides thereof face the permanent magnets with an air gap therebetween to create a pole face. In such a linear motor, in response to the problem of the cogging thrust caused by the end effect, the permanent magnets on one of the stators that are provided side by side are arranged a half-pitch offset with respect to those on the other (see Patent Document 1 and Patent Document 2, for examples).
Another disclosed solution is to offset the positions of teeth, provided on each lateral side of the rotor, along the direction of the movement, instead of offsetting the positions of the permanent magnets on one of the stators, arranged side by side, with respect to those on the other along the direction of the movement (see the Patent Document 1, for example). Furthermore, as another disclosed solution to the issue of the slot cogging thrust, protrusions provided on the heads of the teeth are extended by different lengths in core blocks that are arranged adjacent to each other in an axial direction (for example, see Patent Document 3).
Patent Document 1: Japanese Patent Application Laid-open No. H11-262236 (Pages 3 to 4, FIGS. 1 and 7)                Patent Document 2: Japanese Patent Application Laid-open No. 2003-134790 (Pages 3 to 4, FIGS. 1 and 5)        Patent Document 3: Japanese Patent Application Laid-open No. 2007-60800 (Page 5, FIG. 4)        